Vlad's pool only makes him un-targetable and he cannot cast any other ability while in his pool. There-fore he is not able to be snared, stunned, silenced, ect. So really ya have to look at it from Thresh's abilities not Vlad's. Thresh's lantern ability allows for any close enough ally to gain a shield and grab the lantern as long as they are not snared or stunned and since Vlad cannot be stunned or snared in his pool he can grab the lantern. It's not so much about if he has a hand to grab it but the games mechanics of each skill, in this case Vlad's pool and Thresh's lantern. As long as i've played Vlad with a Thresh it's always been possible and has never been labeled as a bug and probably never will be because the games mechanics are in line with how each skill is supposed to work.